Flexible or “flex” circuits are used in a wide variety of applications where an electrical circuit must bend around corners or be flexed during operation. Flex circuits are thin, light weight, flexible and exhibit high routability. Traditionally, polyimide films have been used as substrates in the manufacture of flex circuits due to their good thermal stability and mechanical strength. Other properties of polyimide films, however, limit the speed or frequency at which electric components mounted thereto can operate.
Liquid crystal polymer (“LCP”) has been developed in recent years as a replacement for polyimide films in flex circuits. LCP is a thermoplastic aromatic polyester which is thermally stable, with an upper use temperature in excess of 250° C. and good inherent flame retardant properties. LCP films, in comparison to polyimide films, have about one-tenth of the moisture uptake and a lower coefficient of humidity expansion. Lower moisture absorption leads to higher frequency signal and data processing. Additionally, LCP films have a lower dielectric constant and a lower loss or dissipation factor over the functional frequency range of 1 kHz to 45 GHz, with negligible moisture effects, compared to polyimide films.
The fabrication of flex circuits with LCP films is expected to lead to their use in more demanding environments where moisture and other contaminants are prevalent. Particularly in such types of applications, the circuit elements applied to the LCP substrate of the flex circuit must be protected from damage. Soldermask coatings, which have been employed to provide protection from moisture and contaminants in polyimide films, have been considered for use with LCP substrates. Additionally, due to the thermoplastic nature of LCP, the application of an LCP film cover layer to an LCP substrate has been proposed as a means of effectively encapsulating circuit elements. There is a need, however, for an efficient and dependable method and apparatus to perform such an encapsulation operation.